The effect of lateral head tilt on horizontal postrotatory nystagmus I and II and the Purkinje effect.

The influence of an active lateral head tilt on postrotatory nystagmus I and II (PI, PII) was explored in normal humans. During postrotatory nystagmus the head was tilted laterally either towards the direction of the previous rotation (ipsilateral tilt) or towards the opposite direction (contralateral tilt) or the head was kept erect. Both ipsi- and contralateral head tilts led to a substantially weaker and shorter PI and PII as compared with the trial without head tilt. The time constants of the decline of PI were shortened to the ones observed in the peripheral nerve of animals. The reduction of PI and PII suggests a position-dependent tonic inhibition of the vestibular storage mechanism in the vestibular nuclei probably by the otoliths. The stronger reduction of time constant and cumulative amplitude of PI after contralateral as compared with ipsilateral head tilt may be explained by different combinations of asymmetric responses in canal and otolith afferents.